Recent findings in Drosophila have shown that 1) a significant proportion of spontaneous mutations are caused by insertions of mobile genetic elements, and 2) certain genetic suppressor systems are mediated through insertions of specific mobile elements. We are investigating the molecular mechanisms of one such suppressor system: recessive mutations at the suppressor-of-sable [su(s)] locus suppress recessive mutations at the vermilion (v) locus that are caused by insertions of the mobile elements 412 and B104 (Project Number Z01 ES 61029-02 LG). DNA sequences of su(s) have been cloned and are being characterized. The insertions of foreign DNA that are associated with 14 su(s) mutant alleles have been localized to a region of DNA that encodes the 5' end of the message. The DNA sequences that give rise to the approximately 5 kb poly A+ su(s) message consists of at least 5 exons interspersed over 8 kb of genomic DNA. Coding sequences from the two largest exons have been ligated into an expression vector to produce fusion proteins against which antibodies can be produced. Antibodies against the su(s) portion of the fusion protein will be recovered and used as probes to identify the location and function of the su(s) protein product within the organism. Wild type su(s) sequences will be introduced into su(s) mutant flies by P element transformation to determine the effect of the su(s) protein product on the biology of mobile element 412.